Forget the meteorites - it was insects that did for the dinosaurs

They were the most imposing and terrifying creatures that have ever walked on the surface of the Earth, but according to a new theory the dinosaurs may have been pushed towards extinction 65m years ago by humble insects.

During the later part of the dinosaurs' dominion over the land, insects underwent an explosion in diversity and in the process dealt a double whammy to the lumbering giants - they spread disease and contributed to a transformation of vegetation which the plant-eating reptiles failed to adapt to.

The hypothesis is laid out in a new book by entomologists George and Roberta Poinar. George Poinar is a professor of zoology at Oregon State University.

"We can't say for certain that insects are the smoking gun, but we believe they were an extremely significant force in the decline of the dinosaurs," said Poinar. "Our research with amber shows that there were evolving, disease-carrying vectors in the Cretaceous [period], and that at least some of the pathogens they carried infected reptiles. This clearly fills in some gaps regarding dinosaur extinctions."

In the gut of one biting insect preserved in amber - fossilised tree sap - from that era, the team has found the pathogen that causes the parasitic disease leishmaniasis, and in another they found a type of malaria parasite that infects birds and lizards. By inspecting fossilised dinosaur faeces, the team also found parasitic microbes that are carried by insects.

These gradually took over from seed ferns, cycads and gingkoes. If herbivorous dinosaurs could not adapt to this new diet they would have gone hungry.

Poinar believes that the most popular theory for the dinosaurs' demise - that a meteorite impact changed the global climate - falls short because the extinction took too long.

"Other geologic and catastrophic events certainly played a role. But by themselves, such events do not explain a process that in reality took a very, very long time, perhaps millions of years. Insects and diseases do provide that explanation."